1. Field of the Invention
This invention relates generally to perpendicular magnetic recording media, and more particularly to a disk with a perpendicular magnetic recording layer for use in magnetic recording hard disk drives.
2. Description of the Related Art
Perpendicular magnetic recording (PMR) disks, wherein the recorded bits are stored in a perpendicular or out-of-plane orientation in the recording layer (RL) on the disks, are used in magnetic recording hard disk drives. One type of material for the RL is a granular ferromagnetic cobalt alloy, such as a CoPtCr alloy, with a hexagonal-close-packed (hcp) crystalline structure having the c-axis oriented substantially out-of-plane or perpendicular to the RL. To induce this growth of the hcp CoPtCr alloy RL, the interlayer (IL) onto which the RL is formed is also an hcp material. Ruthenium (Ru) and certain Ru alloys, such as RuCr, are nonmagnetic hcp materials that are typically used for the IL.
The granular CoPtCr alloy RL should also have a well-isolated fine-grain structure to reduce intergranular exchange coupling, which is responsible for high intrinsic media noise. Enhancement of grain segregation in the cobalt alloy RL is achieved by the addition of oxides, including oxides of Co, Cr, Si, Ta, Ti, Nb and B. These oxides tend to precipitate to the grain boundaries, and together with the elements of the cobalt alloy form nonmagnetic intergranular material. The enhancement of segregation of the magnetic grains in the RL by the additive oxides also controls the size and distribution of the magnetic grains, which is important for achieving high areal density and recording performance.
Thus it is important that future PMR disks have CoPtCr-oxide alloy RLs with very small grains. However, very small magnetic grains can be demagnetized simply from thermal instability or agitation within the magnetized bit (the so-called “superparamagnetic” effect). The thermal stability of a magnetic grain is to a large extent determined by KuV, where Ku is the magneto-crystalline anisotropy of the CoPtCr alloy and V is the volume of the magnetic grain. Thus to avoid thermal instabilities of the stored magnetization, CoPtCr alloys with high Ku are required, especially as the grains become smaller. However, increasing Ku also increases the short-time switching field H0 of the media, which is the field required to reverse the magnetization direction. For most magnetic materials H0 is substantially greater, for example about 1.5 to 2 times greater, than the coercive field or coercivity Hc measured on much longer time-scales. Obviously, the switching field cannot exceed the write field capability of the recording head, which currently is limited to about 12 kOe for perpendicular recording.
Additionally, to improve the writability of the RL, it is desirable for the RL to be formed of at least two ferromagnetically exchange-coupled magnetic layers having different anisotropies to provide a graded anisotropy across the thickness of the RL. This type of graded-anisotropy RL compensates for the variation in write field across the thickness of the RL and non-uniformities in the write field gradient. In CoPtCr alloys a high anisotropy is obtained by increasing the Pt content to about 22-30 atomic percent (at %) and reducing the Cr content to less than about 15 at %. However, decreasing the Cr content in CoPtCr-oxide alloys is known to adversely impact the extent of the oxide segregation boundaries and inhibits the reduction of grain size.
What is needed is a PMR disk with a graded-anisotropy RL formed of at least two CoPtCr-oxide layers wherein the magnetic grain size and the ferromagnetic exchange coupling between the individual layers can be controlled.